This invention relates to a process for selectively extracting lignin from cellulosic materials. The lignin component of lignocellulosic materials is widely recognized as a major limiting factor to rapid degradation and utilization of plant polysaccharides by microorganisms. A major method for complete delignification of biomass practiced by the pulp and paper industry utilizes alkaline pretreatment. In the presence of 5% alkali, at temperatures from 130.degree. to 180.degree. C., the cleavage of ether and carbon bonds readily occurs resulting in the depolymerization and solubilization of the lignin polymer. However, under these conditions, the major portion of the hemicellulose carbohydrates are also solubilized and subsequently degraded to saccharinic acids. These carbohydrate degradation products are not utilized and are toxic to many microorganisms.
The low temperature alkaline treatment of poor quality rough grasses has been shown to increase their nutritive quality as animal feed as shown by Beckman, Chemical Abstracts, 16:765 (1921). However, unless the alkali solubilized lignin is removed, the treatment does not substantially improve the use of the treated grasses in fermentation processes for the production of chemicals or liquid fuels such as ethanol. Furthermore, delignification under these conditions also results in substantial removal of pentosan and uronic acid hemicellulose carbohydrates. Although this type of pretreatment has been shown to markedly enhance the subsequent fermentative utilization of the cellulose remaining, it is not an acceptable delignification method since the hemicellulose carbohydrate fraction removed with the solvent is no longer available as substrate for the fermentation process.
It has been proposed to utilize cold alkaline ethanolic solutions for the isolation of polyuronides from cocks foot grass by Buston, Bio. Chem. J., 28:1028 (1934). It has also been proposed to use these solutions with straws in order to prepare pure hemicelluloses, Norman, Bio. Chem. J., 29:945 (1935). However, the process of Norman was capable of delignifying the straw only one third to one half of acceptable levels. It is believed that one reason for this is that Norman utilized a small amount of acetic acid in the solvent wash. Furthermore, the time period utilized by Norman in the solvent extraction process was insufficient to promote selective extraction of lignin.
Accordingly, it would be highly desirable to provide a solvent extraction process for selectively solubilizing lignin from a cellulosic material while minimizing solubilization of cellulose or hemicellulose. Such a process would be capable of producing a novel delignified cellulosic material which could be utilized, for example, in the fermentative production of ethanol.